Ricin is a highly toxic plant protein that contains two polypeptide chains, an A\chain that enzymatically inhibits ribosomal action and a B\chain that binds the molecule to the cell surface through carbohydrate residues contained in receptors on the cell. Monoclonal antibodies directed against mouse Thy\1.1 antigen have been coupled to two polypeptides that enzymatically inactivate ribosomes and kill the cell that they enter. Such covalently coupled conjugates or immunotoxins are highly specific in the cells that they kill. Cells which differ in only Thy\1 (Thy\1.2) antigen are not specifically killed by Thy\1.1-directed immunotoxins. These results were obtained in vitro using two different plant polypeptides, ricin A\chain and pokeweed antiviral protein. The specific action of the immunotoxins also were demonstrated in vivo. The immunotoxins were capable of preventing the growth of leukemia cells that contained Thy\1.1 antigen but not the growth of leukemia cells that contained Thy\1.2 antigen. Immunotoxins, therefore, may be of use in treatment of certain types of leukemias, as well as other types of cancer. In order to achieve even more selectivity and efficiency of the transport of immunotoxins into the cancer cell once it is bound to the cell surface, we have made new affinity labels to inactivate the carbohydrate binding site in ricin B\chain. This site introduces an element of nonspecificity when ricin is conjugated to Thy\1.1 antibody. This site is specifically inactivated by the photoaffinity label we developed, enabling us to make an immunotoxin whose specificity is determined solely by the antibody, but one that may be able to use transport signals contained on ricin B\chain for more efficient transport than immunotoxins containing Thy\1.1 antibody and ricin A\chain.